I'm a Sri Lankan dear user. As I know It's is not unlawful. But it can be considered as a disrespectful thing or as an insulting to Buddhist religion in Sri Lanka.
The problem was tattoos were not familiar in Sri Lanka. Sri Lankans aka 'Theravada Buddhists' believe that things related to the blessed one (the supreme Buddha) shouldn't be in our body as a tattoo, and shouldn't even be in our bed rooms (where we may do disrespectful things in front of the blessed one. But all of these are not laws just the tradition.
In the case you mentioned, that women had a tattoo of thw blessed one I guess. Some people misunderstood it as that women was trying to insult Buddhist. But she was released immediately.
Sri Lanka doesn't have tough laws related to religion. It's very kind hearted society. So if you are going to visit Sri Lanka just cover those tattoos and then people won't misunderstand you. And kindly explain that you are not insulting Buddhists and It's just a tradition in your country. May the triple gem bless you!